Method of hermetically sealing silicon to a low expansion alloy

ABSTRACT

A surface of silicon is hermetically sealed to a surface of Kovar by coating the surface of silicon with several thin layers of silver, coating the surface of Kovar with a layer of copper, applying a tin-base sealing alloy between the two coated surfaces and then heating the tin-base sealing alloy so as to cause it to fuse to the two coated surfaces.

Unlted States Patent 1151 3,703,306

LaChance 1 Nov. 21, 1972 [54] METHOD OF HERMETICALLY 2,974,761 3/1961 Patrichi ..287/189.365 SEALING SILICON TO A LOW 3,243,072 3/1966 Day ..287/ 189.365 x EXPANSION ALLOY 3,006,069 10/ 1961 Rhoades et al. ..29/473.l 3,025,439 3/1962 Anderson ..29/590 X [72] gsflf Lacham Pasadena 3,418,422 12/1968 Brodham ..317/234 UX 3,462,349 8/1969 Gorgengi ..317/234 UX Assigneoi Xerox Corporation, Stamford, 3,609,471 9/1971 Scace ..317/234 UX Conn.

[22] Filed. Nov. 9, 1970 Primary Examiner-John F. Campbell Appl'. No.: 88,034

Assistant ExaminerRonald J. Shore Attorney-James J. Ralabate, John E. Beck, Franklyn C. Weiss and Irving Keschner [5 7 ABSTRACT A surface of silicon is hermetically sealed to a surface of Kovar by coating the surface of silicon with several thin layers of silver, coating the surface of Kovar with a layer of copper, applying a tin-base sealing alloy between the two coated surfaces and then heating the tin-base sealing alloy so as to cause it to fuse to the two coated surfaces.

5 Claims, 2 Drawing Figures PATENTEIIunvzi m2 i- 11i w.

' INVENTOIL Maniac/Y1K lafkazzce METHOD OF HERMETICALLY SEALING SILICON TO A LOW EXPANSION ALLOY This invention relates to joining silicon to a low expansion alloy. More particularly, this invention relates to hermetically sealing a surface of silicon to a surface of a low expansion alloy.

Silicon has a wide variety of uses and applications. Two of the more important uses for silicon are as a semiconductor material and as an optical material. Frequently, it is either desirable or necessary to hermetically seal an article or device that is made of or coated with silicon to an article or device that is made of a low expansion alloy. As is well known in the art, it is not simple to join metalloid material to metallic material. Some of the known techniques for joining, but not necessarily hermetically sealing, silicon to a low expansion alloy are disclosed in US. Pat. No. 2,856,681, US. Pat. No. 3,031,747 and US. Pat. No. 3,298,093.

It is an object of this invention to provide a new and improved method for joining silicon to a low expansion alloy.

It is another object of this invention to provide a new and improved method for hermetically sealing silicon to a low expansion alloy.

It is still another object of this invention to provide a new and improved method for hermetically sealing a surface of silicon on one body to a surface of a low expansion alloy on another body.

The above and other objects are achieved by joining a surface of silicon to a surface of low expansion alloy according to the method of this invention. Briefly, the method involves preparing the surface of silicon and the surface of the low expansion alloy, then coating the surface of the body of silicon with silver and the surface of the low expansion alloy with copper or other metal which will improve the brazing characteristic of the low expansion alloy, then applying a tin-base sealing alloy between the two coated surfaces and then heating the tin-base sealing alloy so as to cause it to fuseto the two coated surfaces.

Many features and advantages of the invention will become apparent from the following detailed description when taken in conjunction with the drawings wherein:

FIG. 1 is a view partly in elevation and partly in section of a silicon optical window hermetically sealed to a Kovar support member using the method of this invention; and

FIG. 2 is a flow chart illustrating the basic steps of the method of this invention.

Referring now to FIG. 1, there is shown an optical assembly 11. The assembly 11 includes a silicon window '12 mounted in a Kovar support member 13. Kovar, as

is well known in the art, is the trade name of a low expansion alloy made of a mixture of iron, nickel and cobalt. The percentages of iron, nickel and cobalt in the mixture are nominally 55 percent iron, 28 percent nickel and 17 percent cobalt. The surface of the silicon window 12 in contact with the Kovar support member 13 is provided with a coating of silver 14 and the surface of the Kovar support member 13 in contact with the silicon window 12 is provided with a coating of copper 15. A quantity of a tin-silver sealing alloy 16 is disposed between the two coated surfaces and is fused to the two coated surfaces so as to hermetically seal the silicon window 12 to the Kovar support member 13.

The method of assembling the silicon window 12 and the Kovar support member 13 so that the silicon window I2 is hermetically sealed to the Kovar support member 13 is as follows.

First, the surfaces of the silicon window 12 and the Kovar support member 13 which are to be connected to each other are properly prepared. The surface of the silicon window 12 is prepared by first grinding it with a number 9 or finer diamond grit or similar material so as to make it smooth, then swab etching it with a solution of 46 percent HNO 27 percent HF 27 percent Acetic Acid or other similar solution to remove silicon oxide and any other contaminants, then rinsing it first in deionized water or a similar liquid and then in a reagent grade ethanol or other similar solution to remove residue material. The surface of the Kovar support member 13 is prepared by first cleaning it ultrasonically in hot Oakite solution or other similar solution to remove surface contaminants, then rinsing it first in deionized water or a similar liquid and then in ethanol or other similar liquid to remove the Oakite.

Once the two surfaces are properly prepared, the surface of the silicon window 12 is premetallized and the surface of the Kovar support member 13 is preplated. The surface of the silicon window 12 is premetallized in the following manner. First a coating of silver 14 made up of about three to five separate layers of a silver-base slurry or equivalent mixture is applied by a brush or other similar means. Several layers rather than a single layer of silver are necessary to build a coating that is both non-porous and sufficiently thick to prevent it from being dissolved during the subsequent fusing process. After each layer is applied the surface is heated to about 120 C for about 3 to 4 hours and fired at around 600 C for about one hour in air ambient. Then one or more coatings of a 50 percent Pt 50 percent Au or similar metallizing suspension may be applied by brush or other similar means over the multiple layer silver coating. This is done to improve the wettability of the surface. The surface of the Kovar support member 13 is preplated by coating it by electroplating or other'similar means with a layer of copper 15 approximately 0.001 inches thick.

The next step involves applying a tin-base sealing alloy 16 such as a mixture of percent tin and 5 percent silver by weight between the two coated surfaces. Before the tin-base sealing alloy 16 is applied between the two coated surfaces it is degreased in acetone or other similar liquid, rinsed in ethanol or other similar liquid, air dried, dipped into a flux such as Kester Electronic Flux No. 196 and then air dried.

Then the assembly 11 is heated so as to cause the sealing alloy 16 to fuse to the two coated surfaces. This involves heating the assembly 16 in an argon or other protective atmosphere to about 242 C or above the melting point of the sealing alloy and then, with minimum dwell time, causing the assembly to cool rapidly.

It is to be understood that the method described above with respect to hermetically sealing a silicon window and a Kovar support member is only by way of example and that the invention is applicable to items made of silicon other than windows, items made of Kovar other than support members, items made of low expansion alloys other than Kovar and bodies of materials not made of silicon but coated with silicon and not merely bodies made out of silicon.

The basic steps of the method of this invention are shown in the flow chart in FIG. 2.

What is claimed is:

l. A method for forming a hermetic seal between the mating surfaces of a silicon member and a metal alloy comprising the steps of:

a. preparing the mating surfaces of the silicon member and said metal alloy.

b. forming a plurality of metal layers on the surface of said silicon member, each of said layers consisting essentially of silver,

c. coating the metal alloy mating surface with a layer of copper,

d. interposing a sealing alloy between the layered mating surfaces,

e. heating the sealing alloy to a temperature at least equal to the melting point of the sealing alloy, and

f. cooling the contact area of the mating surfaces and the sealing alloy to effect fusing of the sealing alloy to the silver and copper layers.

2. The method as defined in claim 1 wherein said metal alloy comprises Kovar.

3. The method as defined in claim 2 wherein said sealing alloy is heated in an argon atmosphere to a temperature of about 242 C.

4. A hermetically sealed assembly comprising a silicon member, a plurality of metal layers overlying one surface of said silicon member, said metal layers each consisting essentially of silver, a metal alloy member having a copper coating, and a sealing alloy sandwiched between the layered surface of said silicon member and said coated metal alloy member, said sealing alloy being fused to said copper and silver layers.

5. The assembly as defined in claim 4 wherein said metal alloy comprises Kovar. 

1. A method for forming a hermetic seal between the mating surfaces of a silicon member and a metal alloy comprising the steps of: a. preparing the mating surfaces of the silicon member and said metal alloy. b. forming a plurality of metal layers on the surface of said silicon member, each of said layers consisting essentially of silver, c. coating the metal alloy mating surface with a layer of copper, d. interposing a sealing alloy between the layered mating surfaces, e. heating the sealing alloy to a temperature at least equal to the melting point of the sealing alloy, and f. cooling the contact area of the mating surfaces and the sealing alloy to effect fusing of the sealing alloy to the silver and copper layers.
 2. The method as defined in claim 1 wherein said metal alloy comprises Kovar.
 3. The method as defined in claim 2 wherein said sealing alloy is heated in an argon atmosphere to a temperature of about 242* C.
 4. A hermetically sealed assembly comprising a silicon member, a plurality of metal layers overlying one surface of said silicon member, said metal layers each consisting essentially of silver, a metal alloy member having a copper coating, and a sealing alloy sandwiched between the layered surface of said silicon member and said coated metal alloy member, said sealing alloy being fused to said copper and silver layers. 